U.S. patent number 7,858,061 [Application Number 11/300,879] was granted by the patent office on 2010-12-28 for compositions and methods for removing mercury from mercury-containing fluids.
This patent grant is currently assigned to N/A, The United States of America as represented by the Administrator of the United States Environmental Protection Agency. Invention is credited to Yuhong Ju, Timothy C. Keener, Joo Youp Lee, Subhas Sikdar, Rajender S. Varma.
United States Patent |
7,858,061 |
Varma , et al. |
December 28, 2010 |
Compositions and methods for removing mercury from
mercury-containing fluids
Abstract
Oxidative sorbents are provided for adsorbing elemental or
oxidized mercury from mercury-containing fluids such as flue gas
from a coal-burning power utility or the like at a temperature
range of about 50 to 350.degree. C. The method of preparing and
using the oxidative sorbents is also provided. The oxidative
sorbent compositions include one or more silicates capable of
cation exchange with a plurality of active metal cations and their
counter anions. The silicates may include those selected from clays
such as montmorillonite, laumonite, bentonite, Mica, vermiculite
and kaolinite, and from silica gels, natural and synthetic
molecular sieves, zeolites, and ashes from stoker- and pulverized
coal-fired boilers. The one or more oxidative metal halides and/or
sulfates may be selected from the group consisting of CuCl, CuBr,
CuCl.sub.2, CuBr.sub.2, CuSO.sub.4, FeCl.sub.2, FeCl.sub.3,
FeSO.sub.4, Fe.sub.2(SO.sub.4).sub.3, ZnCl.sub.2, ZnBr.sub.2,
NiCl.sub.2, and NiSO.sub.4. The oxidative sorbents may also include
activated carbon.
Inventors: |
Varma; Rajender S. (Cincinnati,
OH), Ju; Yuhong (Cincinnati, OH), Sikdar; Subhas
(Blue Ash, OH), Lee; Joo Youp (Cincinnati, OH), Keener;
Timothy C. (Cincinnati, OH) |
Assignee: |
The United States of America as
represented by the Administrator of the United States Environmental
Protection Agency (Washington, DC)
N/A (N/A)
|
Family
ID: |
38173744 |
Appl.
No.: |
11/300,879 |
Filed: |
December 15, 2005 |
Prior Publication Data
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|
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Document
Identifier |
Publication Date |
|
US 20070140940 A1 |
Jun 21, 2007 |
|
Current U.S.
Class: |
423/215.5;
588/313; 502/400 |
Current CPC
Class: |
B01D
53/64 (20130101); B01D 2257/602 (20130101); B01D
2253/106 (20130101) |
Current International
Class: |
B01D
45/00 (20060101) |
Field of
Search: |
;502/405-407,410,413
;588/313,400,412 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
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|
Primary Examiner: Vanoy; Timothy C
Assistant Examiner: Rump; Richard M
Claims
We claim:
1. An oxidative sorbent composition for the substantial removal of
mercury from mercury-containing fluids comprising: a matrix
material selected from one or more silicates; at least one metal
halide impregnated in the matrix material for oxidizing Hg.sup.0
present in the mercury-containing fluids to one of Hg(I) and
Hg(II); and about 1% to about 30% activated carbon.
2. The oxidative sorbent composition of claim 1, wherein the at
least one metal halide is selected from the group consisting of
CuCl, CuBr, CuCl.sub.2, CuBr.sub.2, FeCl.sub.2, FeCl.sub.3,
ZnCl.sub.2, ZnBr.sub.2, and NiCl.sub.2.
3. The oxidative sorbent composition of claim 1, wherein the one or
more silicates are selected from the group consisting of clays such
as montmorillonite, laumonite, bentonite, mica, vermiculite and
kaolinite, and from silica gels, natural and synthetic molecular
sieves, zeolites, and ashes from stoker and pulverized coal-fired
boilers.
4. The oxidative sorbent composition of claim 1, further comprising
activated carbon.
5. The oxidative sorbent composition of claim 1, wherein the
mercury-containing fluids comprise acidic flue gas.
6. The oxidative sorbent composition of claim 1, wherein the
composition comprises about 1 to about 35% (wt) of at least one
metal halide and from about 65 to about 99% (wt) matrix
material.
7. The oxidative sorbent composition of claim 1, wherein the matrix
material has a particle size range of about one to about 100
microns.
8. The oxidative sorbent composition of claim 7, wherein the matrix
material has an average particle size of less than about 32
microns.
9. A method for preparation of an oxidative sorbent composition for
use in substantially removing mercury from fluids, comprising the
steps of: contacting one or more silicates with at least one metal
halide, wherein the at least one metal halide is in a solution, to
make metal-impregnated silicates for oxidizing Hg.sup.0 to one of
Hg (I) and Hg(II); adding activated carbon to the metal-impregnated
silicates; and removing the solution from the oxidative sorbent
composition; wherein the at least one metal halide is selected from
the group consisting of CuCl, CuBr, CuCl.sub.2, CuBr.sub.2,
FeCl.sub.2, FeCl.sub.3, ZnCl.sub.2, ZnBr.sub.2, and NiCl.sub.2; and
wherein the one or more silicates are selected from the group
consisting of clays such as montmorillonite, laumonite, bentonite,
Mica, vermiculite and kaolinite, and from silica gels, natural and
synthetic molecular sieves, zeolites, and ashes from stoker--and
pulverized coal-fired boilers.
10. The method of claim 9, wherein the at least one metal halide is
only CuCl.sub.2.
11. The method of claim 9, wherein the one or more silicates is
montmorillonite.
12. The method of claim 9, wherein the solution is an aqueous
solution of about 6.25% to about 20% of the at least one metal
halide in water.
13. The method of claim 9, wherein the solution is an acetone
solution of about 6.25% to about 20% of the at least one metal
halide in acetone.
14. The method of claim 9, wherein the one or more silicates have
an average particle size ranging from about 1 to about 100
micrometers.
15. A method to capture mercury from mercury-containing fluid,
comprising the steps of: contacting the mercury-containing fluid
with an oxidative sorbent, the oxidative sorbent comprising one or
more silicates impregnated with at least one metal halide for
oxidizing Hg.sup.0 to one of Hg(I) and Hg(II); oxidizing Hg.sup.0
present in the mercury-containing fluid to one of Hg(I) and Hg(II)
with the metal halide; and capturing the Hg(I) and Hg(II) onto at
least one of the silicates of the oxidative sorbent; wherein the at
least one metal halide is selected from the group consisting of
CuCl, CuBr, CuCl.sub.2, CuBr.sub.2, FeCl.sub.2, FeCl.sub.3,
ZnCl.sub.2, ZnBr.sub.2, and NiCl.sub.2; and wherein the one or more
silicates are selected from the group consisting of clays such as
montmorillonite, laumonite, bentonite, Mica, vermiculite and
kaolinite, and from silica gels, natural and synthetic molecular
sieves, zeolites, and ashes from stoker--and pulverized coal-fired
boilers.
16. The method of claim 15, wherein the oxidative sorbent further
comprises activated carbon and wherein the silicate is
montmorillonite clay and the metal halide is CuCl.sub.2.
17. A method to capture mercury from a flue gas stream containing
elemental and ionic mercury, comprising the steps of: injecting a
porous oxidative sorbent into the flue gas stream, wherein the
porous oxidative sorbent comprises: at least one metal halide
selected from the group consisting of CuCl, CuBr, CuCl.sub.2,
CuBr.sub.2, FeCl.sub.2, FeCl.sub.3, ZnCl.sub.2, ZnBr.sub.2, and
NiCl.sub.2 for oxidizing Hg.sup.0 to one of Hg(I) and Hg(II); and a
matrix material selected from one or more silicates selected from
the group consisting of clays such as montmorillonite, laumonite,
bentonite, Mica, vermiculite and kaolinite, and from silica gels,
natural and synthetic molecular sieves, zeolites, and ashes from
stoker--and pulverized coal-fired boilers; oxidizing Hg.sup.0
present in the flue gas stream to one of Hg(I) and Hg(II) with the
metal halide; capturing the oxidized mercury onto the porous
oxidative sorbent when the oxidative sorbent is exposed to the flue
gas stream; and removing and disposing of the spent oxidative
sorbent.
18. A process for treating the surface of one or more silicates for
ion exchange comprising: contacting the surface of montmorillonite
clay with CuCl.sub.2 in acetone solution, wherein the CuCl.sub.2 is
used to oxidize Hg.sup.0 present in a flue gas stream to one of
Hg(I) and Hg(II).
Description
FIELD OF THE INVENTION
This invention relates generally to pollution control and more
specifically, to adsorbents which substantially reduce the amount
of mercury released into the environment by coal-fired utility
plants and from other sources.
BACKGROUND OF THE INVENTION
Mercury and its compounds are significant environmental pollutants
and major threats to human life and natural ecosystems. Mercury is
of significant environmental concern because of its toxicity,
persistence in the environment, and bioaccumulation in the food
chain. The toxicity of soluble Hg ions and elemental Hg even in
very dilute concentrations has been widely reported in the
literature. Mercury is released readily into the environment from
natural and anthropogenic sources. Because of its physical and
chemical properties, mercury can also be transported regionally
through various environmental cycles. Mercury Study Report to
Congress, "Volume VIII: An Evaluation of Mercury Control
Technologies and Costs," U.S. Environmental Protection Agency,
EPA452/R-97-010, December, 1997..sup.1 Atmospheric deposition of
mercury is reported to be the primary cause of elevated mercury
levels in fish which is a potential threat to pregnant women and
young children. 2004 EPA and FDA Advice, "What You Need to Know
About Mercury in Fish and Shellfish," EPA-823-R-04-005, March,
2004..sup.2
The annual global mercury emission is estimated at 5000 tons.
Miller, S. J., et al., "Laboratory-Scale Investigation of Oxidative
sorbents for Mercury Control," 94-RA 114A.01, presented at the 87th
Annual Air and Waste Management Meeting, Cincinnati, Ohio, Jun.
19-24, 1994..sup.3
The United States accounts for approximately 3% of such mercury
emissions although this persistent pollutant travels globally via
jet stream and gets converted to methyl mercury in the environment
with high neurodevelopment toxicity. In the United States,
coal-fired power utility plants are the biggest source of mercury
emissions into the air, emitting a total of about fifty metric tons
of mercury into the atmosphere annually, which is approximately
thirty-three percent of all mercury emissions from the United
States. Coal-fired combustion flue gas streams are of particular
concern because of their composition that includes trace amounts of
acid gases, such as SO.sub.2, NOx, and HCl plus CO.sub.2 and oxygen
contents. Other sources of mercury emissions may include the
chlor-alkali industry, metal sulfide or smelting, gold refining,
cement production, fossil fuel combustion and incineration of
sewage sludge or municipal garbage or the like.
The major chemical forms of the metal in the combustion flue gases
are the elemental Hg.sup.0 (zerovalent) and the oxidized mercury,
HgCl.sub.2, Hg(I) and Hg(II). Mercury vapor, Hg.sup.0, is found
predominantly in coal-fired boiler flue gas. Mercury can also be
bound to fly ash in the flue gas. Mercury speciation (elemental or
oxidized) and concentration is dependent on the source (e.g. the
characteristics of the fuel being burned), process conditions and
the constituents in the ensuing gas streams (e.g., Cl.sub.2, HCl,
SO.sub.2, NO.sub.x). The thermodynamically stable predominant form
of mercury in the flue gases from coal-fired utilities is the
elemental one (Hg.sup.0). However, the oxidized HgCl.sub.2 may be
the major species from waste incinerators. Unlike the oxidized
forms, the metal in the zero valent state is difficult to remove
due its high volatility and low water solubility.
There is no currently available control method that efficiently
collects all mercury species present in the flue gas stream. The
existing mercury removal technologies involve scrubbing solution as
in a wet flue gas desulfurization system, filtration and other
inertial methods, electrostatic precipitation, and activated carbon
based sorbents and a few other types of sorbents. For example,
phyllosilicate mineral based sorbent has been described using a
polyvalent metal sulfide prepared by ion exchange of tin, iron,
titanium, zirconium and molybdenum to the support, and sequentially
controlled addition of sulfide ions to the silicate substrate.
However, the preparation and regeneration is tedious, costly, and
dangerous making this technology unlikely to be commercialized.
Sorbent injection is one of the most promising technologies for
application to the utility industry as virtually all coal-fired
boilers are equipped with either an electrostatic precipitator
(ESP) or a baghouse. Among various sorbents tested under the
Department of Energy (DOE)'s field testing program, the most widely
tested and promising sorbent is found to be activated carbon which
has displayed the capability of capturing both elemental and
oxidized mercury from flue gas streams. However, activated carbon
has the following limitations: (1) activated carbon is expensive
(e.g., Norit DARCO FGD activated carbon, DOE's benchmark sorbent,
costs $0.42/lb); (2) it requires a very high carbon-to-mercury mass
ratio (3,000-100,000) especially in flue gases with low HCl content
such as subbituminous and lignite coals, and cannot achieve
90%+mercury removal. Mercury Study Report to Congress; EPA
452/R-97-003; U.S. Environmental Protection Agency, Office of Air
Quality Planning and Standards: Washington, D.C., December
1997..sup.4; and (3) it degrades the quality of captured fly ash
and thereby adversely impacts its sale as a pozzolan additive.
Feeley, T. J.; Brickett, L. A.; O'Palko, B. A.; Murphy, J. T. Field
Testing of Mercury Control Technologies for Coal-Fired Power
Plants. Procd. of DOE/NETL's Mercury Control Technology R&D
Program Review Meeting, Pittsburgh, Pa., Jul. 12-14,
2005..sup.5
Accordingly, there has been a need for novel oxidative sorbent
compositions and methods to substantially reduce mercury emissions
into the environment. There has been a need for novel oxidative
sorbent compositions and method which efficiently and economically
substantially reduce mercury in mercury containing fluids such as
vapor mercury, the elemental form of mercury, from flue gas while
preserving the quality of fly ash. Such oxidative sorbents and
methods are needed to substantially reduce the total cost of
mercury control technology. Additionally, novel compositions and
methods are needed to reduce the amount of oxidative sorbent and
oxidative sorbent injection equipment needed, and reduce costs for
handling and disposing of spent oxidative sorbent. There is a still
further need for efficient oxidative sorbent compositions that
exhibit high adsorption capacity and can tolerate the presence of
acidic gases. There is an additional need for novel oxidative
sorbent compositions and methods that are less expensive and more
efficient than activated carbon alone at removing vapor mercury
from flue gas and may be used in combination with other
mercury-removal technologies. The present invention fulfills these
needs and provides other related advantages.
SUMMARY OF THE INVENTION
The present invention is concerned with an oxidative sorbent
composition, a method of making a oxidative sorbent composition,
and a method of using the oxidative sorbent composition for the
substantial removal of mercury from a mercury-containing fluid. As
used herein, the term "fluid" denotes gas, liquid, vapor, and
combinations thereof.
The oxidative sorbent composition generally comprises one or more
silicates capable of cation exchange with active metal cations, a
plurality of active metal cations presenting at the material
surface, and a plurality of counter anions. The sorbent composition
may further comprise activated carbon.
The one or more silicates may be natural or synthetic. The one or
more silicates may be selected from the group consisting of clays
such as montmorillonite, laumonite, bentonite, Mica, vermiculite
and kaolinite, and from silica gels, natural and synthetic
molecular sieves, zeolites, and ashes from stoker- and pulverized
coal-fired boilers. The particle size of the preferred silicates
used ranges from about 1 to about 100 micrometers
The preferred active metal cations may be selected from the group
consisting of Cu(I), Cu(II), Fe(II), Fe(III), Ni(II) and Zn(II) and
combinations thereof (a monovalent copper ion, a bivalent copper
ion, a bivalent iron ion, a trivalent iron ion, a bivalent nickel
ion, a bivalent zinc ion, and their counter anions chloride
(Cl.sup.-1), bromide (Br.sup.-1), and sulfate
(SO.sub.4.sup.-2).
It is believed that the oxidative sorbent composition functions to
facilitate mercury removal from the mercury-containing fluid using
oxidative reactions of elemental mercury--Hg.sup.0 in the
temperature range of about 50 to about 350.degree. C. and
sequential adsorption of Hg(I) or Hg(II) on the surfaces of the one
or more silicates. Zero valent Hg is relatively more difficult to
be adsorbed than Hg(I) and Hg(II) due to its very low affinity with
other materials.
Generally, the immobilization of metal halides and/or sulfates is
accomplished on the surface of the silicates by suspending,
grinding or otherwise contacting the metal halides/sulfates with
the one or more silicates to produce the metal-impregnated
silicates which can be then collected as powdered solid sorbents.
Activated carbon may be added to the metal-impregnated
silicates.
The method of using the novel oxidative sorbent compositions for
the capture of mercury from a flue gas stream is also provided and
comprises the steps of injecting a porous oxidative sorbent into
the flue gas stream, capturing the mercury onto the porous
oxidative sorbent when the oxidative sorbent is exposed to the flue
gas stream, and removing and disposing of the spent oxidative
sorbent under a set of removal conditions that meets federal and
state disposal criteria and that includes stabilization of the
spent sorbent by known methods and removal of fly ash. The term
"spent oxidative sorbent" as used herein means sorbent that
includes captured mercury.
Other features and advantages of the present invention will become
apparent from the following more detailed description, taken in
conjunction with the accompanying drawings which illustrate, by way
of example, the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings illustrate the invention. In such
drawings:
FIG. 1 is a schematic of a lab-scale fixed-bed system used for
Hg(0) adsorption tests in N.sub.2 flow with an online mercury
analyzer for screening purpose;
FIG. 2 is a graph illustrating the breakthrough curves for some of
the synthetic oxidative sorbents tested in dry N.sub.2 using the
system in FIG. 1 at 140.degree. C.;
FIG. 3 is another schematic of a fixed-bed system for tests under
simulated flue gas conditions;
FIG. 4 illustrates mercury speciation analysis results obtained
using the Ontario Hydro Method through 1-hr fixed-bed tests under a
simulated flue gas of the subbituminous and lignite coals; and
FIG. 5 illustrates the adsorption capacities for three different
oxidative sorbents obtained through 1-hour fixed-bed tests.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is concerned with an oxidative sorbent
composition, a method of making an oxidative sorbent composition,
and a method of using the oxidative sorbent composition for the
substantial removal of mercury from a mercury-containing fluid.
In accordance with a first embodiment of the present invention, a
novel oxidative sorbent composition suitable for removing mercury
from mercury-containing fluids is provided. As used herein, the
term "fluid" denotes gas, liquid, vapor, and combinations thereof.
Coal-fired flue gas is one such gas. The oxidative sorbent
composition generally comprises one or more silicates capable of
cation exchange with active metal cations, a plurality of active
metal cations presenting at the material surface, and a plurality
of counter anions. The oxidative sorbent composition may further
comprise activated carbon.
As used herein, the one or more silicates may be a natural or
synthetic silicate selected from the group consisting of clays such
as montmorillonite, laumonite, bentonite, Mica, vermiculite and
kaolinite, and from silica gels, natural and synthetic molecular
sieves, zeolites, and ashes from stoker- and pulverized coal-fired
boilers. Clays have been already been studied for catalytic
applications because they occur abundantly in nature and because of
their high surface area in the range of approximately 50 to 350
m.sup.2/g, and adsorptive and ion-exchange properties. Moreover,
clays are relatively inexpensive. In addition, fly ashes from
stoker fired boilers as well as from pulverized coal-fired boilers,
including ashes from the combustion of bituminous coals,
subbituminous coals and lignite coals can be used as substrate
materials. The particle size of the preferred silicates ranges from
about 1 to about 100 micrometers, which makes them easier to be
collected after adsorption in an electrostatic precipitator (ESP)
or fabric filter and minimizes the amount of needed oxidative
sorbent. Fly ashes may be larger, up to about 200 microns. The
average particle size of fly ash is about 20 microns.
The preferred active metal cations may be selected from the group
consisting of Cu(I), Cu(II), Fe(II), Fe(III), Ni(II) and Zn(II) and
combinations thereof (a monovalent copper ion, a bivalent copper
ion, a bivalent iron ion, a trivalent iron ion, a bivalent nickel
ion, a bivalent zinc ion, and their counter anions chloride
(Cl.sup.-1), bromide (Br.sup.-1), and sulfate (SO.sub.4.sup.-2).
Chloride is the most common and economical choice as the counter
ion for oxidative metal species such as Fe(III) and Cu(II) and has
already been investigated to enhance the mercury adsorption
capacity of activated carbon by chemisorptions. Spent etching
solution containing CuCl.sub.2 from the Printed Circuit Board
industry in various weight compositions doped on inexpensive clay
as hereinafter described provides an efficient and inexpensive
oxidative sorbent to substantially reduce mercury in
mercury-containing fluids.
The metal cations may be immobilized on the porous surface of a
host silicate(s) by ion exchange. The silicates have a layered
structure and immobilized active metal salts, with Cu(I), Cu(II),
Fe(II), Fe(III), Ni(II) and Zn(II) on their surface.
The activated carbon may be added in an amount from about 1 to
about 30% of the metal-impregnated silicates, preferably about 10%
to greatly enhance mercury adsorption. The activated carbon (FGD)
is available from Norit, Marshall, Tex.
While not wishing to be bound by any one theory, it is believed
that the oxidative sorbent composition functions to facilitate
removal of the mercury from the mercury-containing fluid using
oxidative reactions of elemental mercury--Hg.sup.0 in the
temperature range of about 50.degree. C. to about 350.degree. C.
Higher valent metal species can be doped on the surface of porous
silicates and function as oxidants to oxidize Hg.sup.0 in the
temperature range of about 50-350.degree. C.; oxidized forms of Hg,
e.g. Hg(I) and Hg(II) can be more easily captured on the porous
surface of the one or more silicates.
Active oxidative metal halides and/or metal sulfates may be
impregnated onto the silicate mineral supports to oxidize Hg(0) to
Hg(I) or Hg(II), and to capture the oxidized Hg species by
adsorption on the porous surface of the one or more silicates with
or without the activated carbon. It is postulated that the chemical
reaction/adsorption is responsible for the oxidation of elemental
mercury and the subsequent adsorption of the oxidized forms of
mercury on the porous surface of the silicates.
A method for making the novel oxidative sorbent composition is also
provided. Generally, the immobilization of metal halides and/or
sulfates is accomplished on the surface of the mineral oxide by
suspending, grinding or otherwise contacting the metal
halides/sulfates with the one or more silicates to produce the
metal impregnated silicates which can be then collected as powdered
solid oxidative sorbents. Activated carbon may be added to the
metal impregnated silicates. Exemplary preparation is as
follows:
1. Doping the metal halides and/or sulfates (including CuCl, CuBr,
CuCl.sub.2, CuBr.sub.2, CuSO.sub.4, FeCl.sub.2, FeCl.sub.3,
FeSO.sub.4, Fe.sub.2(SO.sub.4).sub.3, ZnCl.sub.2, ZnBr.sub.2,
NiCl.sub.2, NiSO.sub.4) to powdered minerals (silicates) (particle
size ranging from about one to about one hundred micrometers) via
impregnation by contacting the suspension of the silicate(s) with
metal halides/sulfate in the aqueous phase solution (1-35% w/w
metal halides/sulfate on mineral).
Preparation of a 5% Copper Chloride Impregnated Oxidative
Sorbent
Add 0.50 g of copper chloride CuCl.sub.2 (Aldrich Chemical. Co.,
Milwaukee, Wis., USA) to a suspension of 4.95 g of montmorillonite
K 10 clay (Aldrich Chemical. Co. Milwaukee, Wis., USA) mixed in 80
mL of distilled water. Maintain the mixture at room temperature
(.about.20.degree. C.) for 4 hours with continuous stirring. The
resulting suspension was filtered and the solid material dried at
100.degree. C. under vacuum for 16 hrs to prepare 5% copper
chloride impregnated oxidative sorbent (CuCl.sub.2-MK10).
2. Doping the metal halides and/or sulfates (including CuCl, CuBr,
CuCl.sub.2, CuBr2, CuSO.sub.4, FeCl.sub.2, FeCl.sub.3, FeSO.sub.4,
Fe.sub.2(SO.sub.4).sub.3, ZnCl.sub.2, ZnBr.sub.2, NiCl.sub.2,
NiSO.sub.4) to powdered minerals (particle size ranging from about
one to about one hundred micrometers) by contacting the suspension
of one or more silicates with metal halides/sulfate in acetone
solution (Aldrich Chemical Co., Milwaukee, Wis., USA, A.C.S. grade)
(1-35% w/w metal halides/sulfate on mineral).
Preparation of a 5% Copper Chloride Impregnated Oxidative
Sorbent
Add 0.50 g of copper chloride CuCl.sub.2 (Aldrich Chemical. Co.,
Milwaukee, Wis., USA) to the suspension of 4.95 g montmorillonite K
10 clay (Aldrich Chemical Co. Milwaukee, Wis., USA) in 80 mL of
acetone, the mixture was kept at 50.degree. C. for 2 hrs. The
solvent was then removed from the resulting suspension below
50.degree. C. using a rotary evaporator (BUCHI Rotavapor R-200,
BUCHI Laboratory Equipment, Switzerland). 5% copper chloride
impregnated montmorillonite clay was obtained as light blue
solid.
Preparation of a 5% Copper Chloride/Copper Sulfate (1/1)
Impregnated Kaolin
Add 0.25 g of copper chloride CuCl.sub.2 (Aldrich Chemical. Co.,
Milwaukee, Wis., USA), and 0.25 g of copper sulfate CuSO.sub.4
(Aldrich Chemical. Co., Milwaukee, Wis., USA) to a suspension of
4.95 g of Kaolin (Fisher Scientific. Chicago, USA) in 80 mL of
acetone and keeping the mixture at 50.degree. C. for 2 hrs. The
solvent was then removed from the resulting suspension below
50.degree. C. using a rotary evaporator (BUCHI Rotavapor R-200,
BUCHI Laboratory Equipment, Switzerland). 5% copper chloride/copper
sulfate (1/1) impregnated kaolin was obtained as light blue
solid.
3. Dry impregnation (or physical mixing)
Preparation of a 5% Copper Chloride Impregnated Oxidative
Sorbent
Mix 0.50 g of copper chloride CuCl.sub.2 (Aldrich Chemical. Co.
Milwaukee, Wis., USA) with 4.95 g montmorillonite K 10 clay using a
mortar and pestle or the like and grinding together thoroughly with
the pestle to obtain 5% copper chloride impregnated montmorillonite
clay as the oxidative sorbent.
Although the preparation of a 5% copper chloride impregnated
montmorillonnite K 10 oxidative sorbent and a 5% copper
chloride/copper sulfate (1/1) impregnated kaolin oxidative sorbent
have been described, it is to be appreciated that an oxidative
sorbent composition comprised of other silicates and other metal
halides/sulfates and combinations thereof may be generally prepared
by combining the support components, described above, together in
appropriate proportions, described above, by any suitable method or
manner known in the art which provides impregnation of the one or
more silicates by one or more metal halides/metal sulfate. A
preferred oxidative sorbent has an average surface area in the
range of about 50 to about 350 m.sup.2/g, preferably about 250
m.sup.2/g.
Activated carbon in an amount of about 1% to about 30% of the metal
impregnated silicates may be added to the metal-impregnated
silicates, preferably about 10% to about 30%. Although addition of
the activated carbon to metal-impregnated silicates is described,
it is to be appreciated that the activated carbon may be added to
the silicate(s) at the same time as the metal halides/sulfates. The
method of using the novel oxidative sorbent compositions for
mercury capture from mercury-containing fluid is also provided and
comprises the steps of: Injecting a oxidative sorbent into the
mercury-containing fluid; Capturing the mercury onto the porous
oxidative sorbent when the oxidative sorbent is exposed to the
mercury-containing fluid; and Removing and disposing of the spent
oxidative sorbent. The capturing step includes oxidizing the
elemental mercury (Hg(0)) inside the porous oxidative sorbent and
capturing the oxidized mercury (Hg(+) and/or Hg(2+) onto the
oxidative sorbent when the oxidative sorbent is exposed to the flue
gas stream.
The contacting of the mercury-containing fluid and oxidative
sorbent composition is carried out under a set of removal
conditions that include injection of the oxidative sorbent into a
flue gas stream from -100 to +100 inches of water static pressure,
gas velocities from 1 to 100 ft/s, and gas-sorbent residence times
from 0 to 60 min. The removal conditions preferably include a
temperature in the range of about 50-350.degree. C., more
preferably about 140.degree. C. for the best mercury removal. When
the oxidative sorbent composition is contacted with the mercury
containing fluid such as a flue gas stream, a significant amount of
elemental and/or oxidized mercury present in the mercury containing
fluid is removed from such a fluid. After separation of the spent
oxidative sorbent from the fluid effluent of, for example, the coal
burning facility, the spent oxidative sorbent is disposed of in a
manner which meets federal and/or state disposal regulations.
The mercury capture performances for the oxidative sorbents
disclosed herein are summarized as follows:
1. Lab-Scale Fixed-Bed Tests in N.sub.2 Flow
A lab-scale fixed-bed system as shown in FIG. 1 was used to
identify potential oxidative sorbent candidates in N.sub.2 flow.
The effluent Hg(0) concentrations at the outlet of the reactor were
monitored by an online mercury analyzer, which can detect only
Hg(0) and cannot detect any oxidized mercury. Therefore, the Hg(0)
concentration differences at the inlet and outlet of the reactor
may result from adsorption onto an oxidative sorbent or
oxidation.
FIG. 2 shows the breakthrough curves of some of the synthetic
oxidative sorbents tested in N.sub.2 flow at 140.degree. C. Since
the breakthrough curves show effluent Hg(0) concentrations with
respect to time, good oxidative sorbent candidates are required to
have low outlet Hg(0) concentrations over a long period of time.
The oxidative sorbents shown in FIG. 2 demonstrate moderate (e.g.,
25% ZnCl.sub.2-MK10) to excellent (e.g., 25% CuCl.sub.2-MK10)
capacities of Hg(0) adsorption/oxidation.
2. Lab-Scale Fixed-Bed Tests in a Simulated Flue Gas Flow
Oxidative sorbents were evaluated at 140.degree. C. in a simulated
flue gas system as shown in FIG. 3. The simulated flue gas
representing the combustion of subbituminous and lignite coals
consisted of 3% (v) O.sub.2, 12% (v) CO.sub.2, 7% (v) H.sub.2O, 500
ppmv SO.sub.2, 200 ppmv NO, and 9 ppbv Hg(0) balanced with
N.sub.2.
All of the tests were conducted during an hour at 140.degree. C. in
the fixed-bed system, and their results are summarized in Table 1.
The mass balance closure for all runs was in a reasonably
acceptable range (87-106%), and the amount of mercury captured in
spent oxidative sorbents was determined after performing the
digestion procedures described in the Ontario Hydro Method. ASTM
Method D6784-02, "Standard Test Method for Elemental, Oxidized,
Particle-bound and Total Mercury in Flue Gas Generated from
Coal-Fired Stationary Sources (Ontario Hydro Method)",.sup.6
incorporated by reference herein. HCl gas was not added to the
simulated flue gas for all the runs in order to eliminate the
well-known heterogeneous mercury oxidation (mercury adsorption on
activated carbon after in-situ HCl gas impregnation on activated
carbon). Carey, T. R.; Hargrove Jr., O. W.; Richardson, C. F.;
Chang, R.; Meserole, F. B. Factors Affecting Mercury Control in
Utility Flue Gas Using Activated Carbon. J. Air & Waste Manage.
Assoc. 1998, 48, 1166..sup.7 However, the performance of the
oxidative sorbent proved not to be affected by an addition of HCl
gas from further testing.
The oxidative sorbent, CuCl.sub.2.2H.sub.2O-MK10, which showed the
best performance in N.sub.2 flow, was evaluated at 140.degree. C.
in Run 1. It showed 1% of mercury capture in the oxidative sorbent,
and approximately 74% of the mercury emissions from the bed were
captured as oxidized mercury in the second filter placed in a
filter holder, a water condensation impinger, and KCl solution
impingers. Run 1 showed that mercury adsorption in the oxidative
sorbent was relatively small (approximately 10% of total Hg(0)
injected into the system), and a significant majority of the inlet
Hg(0) vapor was converted to the oxidized mercury form and adsorbed
onto the solid phase (filter) or absorbed into the aqueous phase
bubbler where water or KCl solution was placed. While 11% of Hg(0)
was captured, most of the influent Hg(0) was found to be converted
to oxidized mercury. In terms of Hg(0) removal, 86% of Hg(0) was
removed. Therefore, the novel oxidative sorbent was found to be an
excellent Hg(0) oxidant.
In Run 2, Norit FGD activated carbon (DOE's benchmark oxidative
sorbent) was tested in order to determine its Hg(0) adsorption
capacity in the absence of HCl in the gas phase. These tests showed
26% sorption capacity and negligible oxidation capability. In
earlier tests, activated carbon did not work well for Hg(0) removal
without HCl gas in any type of simulated flue gases. These results
in the absence of HCl gas corroborate low mercury removal observed
from the flue gases of PRB subbituminous and lignite coals in the
DOE's Mercury Control Field Testing Program.sup.5 and another
fixed-bed study.sup.7.
Runs 3 and 4 were performed to examine the possibility of capturing
the oxidized mercury created from the use of the novel oxidative
sorbent by the in-situ adsorption onto activated carbon. A stannous
chloride (a reducing agent, SnCl.sub.2) solution was used for Run 3
for total mercury analysis after the fixed-bed reactor so that all
the mercury emitted after the bed would be converted to Hg(0), and
could be collected in the downstream KMnO.sub.4 solution impingers.
Run 3 used 20% (10 mg) of the FGD activated carbon after uniformly
mixing it with 80% (40 mg) of the 10% CuCl.sub.2.2H.sub.2O-MK10 in
6 g of sand. Results showed that almost all Hg(0) (98% of the total
87% recovered mercury from all impinger solutions, and digestions
of filters and solids) was captured in the mixture of the two
materials (FGD activated carbon and 10% CuCl.sub.2.2H.sub.2O-MK10)
with 20% addition of the FGD activated carbon. In Run 4, the amount
of FGD activated carbon was reduced to the half that of Run 3, 10%
(5 mg), and was tested under the same conditions. The 10% addition
of the activated carbon also demonstrated almost the same
performance in Hg(0) removal as that of Run 3 under the same test
conditions.
TABLE-US-00001 TABLE 1 1-hr testing results in a fixed-bed reactor
at 140.degree. C. Loading Hg from spent Hg from Mass balance (mg
oxidative oxidative 2.sup.nd filter in Hg in water Hg in closure
based Oxidative sorbent in 6 sorbent + filter condensation tubing
Hg in KCl Hg in on inlet Hg Run sorbent g sand) filter (%) holder
(%) (%) (%) (%) KMnO.sub.4 (%) (%) 1 A** 50 11 19 28 0 27 14 100 2
AC** 50 26 N/A 0 0 0.5 73 106 3 A + AC* 40 + 10 98 N/A 0 0 N/A 2 87
4 A + AC** 45 + 5 98 1 0 0 1 2 90 (Note) A = 10%
CuCl.sub.2.cndot.2H.sub.2O-MK10 AC = Norit's FGD activated carbon
*Impinger configuration for total mercury analysis: 2 SnCl.sub.2
.fwdarw. 1 water trap .fwdarw. 3 KMnO.sub.4 **Impinger
configuration for mercury speciation analysis: 2 KCl .fwdarw. 1
water trap .fwdarw. 3 KMnO.sub.4
From the above test results, the novel oxidative sorbent proved to
exhibit excellent performance and selectivity in removing Hg(0).
The mercury speciation results obtained using the Ontario Hydro
Method in Table 1 are illustrated in FIG. 4. It is anticipated that
injection of the novel oxidative sorbent in conjunction with
activated carbon could achieve 90%+mercury removal in a
cost-effective way, especially for flue gases with relatively high
Hg(0) content.
An adsorption capacity of each oxidative sorbent was determined
using the digestion procedure described in the Ontario Hydro
Method, and is shown in FIG. 5. It shows that the adsorption
capacity of the oxidative sorbent can be greatly enhanced by the
addition of FGD activated carbon. These results obtained from the
fixed-bed tests are expected to be replicated through oxidative
sorbent injection prior to a fabric filter system in a large scale
system.
The injection of the oxidative sorbent in conjunction with
activated carbon can achieve 90%+mercury removal regardless of the
mercury speciation present in the flue gas. As the novel oxidative
sorbent has been shown to have excellent oxidant capability, it may
be injected with activated carbon to achieve 90%+mercury removal
and collected in particulate control devices such as ESPs or fabric
filters. As an alternative, only the noncarbonaceous oxidative
sorbent may be injected prior to a wet scrubber if it is available,
and oxidized mercury can be readily removed in the wet scrubber. In
this scenario, the oxidative sorbent would be injected upstream of
the existing ESP and captured, while the resultant highly oxidized
mercury would be absorbed into the downstream wet scrubber.
Therefore, the method of using the disclosed oxidative sorbents may
be in combination with conventional air pollution control
technologies such as electrostatic precipitation, wet scrubbing,
filtration and other inertial methods in order to utilize the
existing air pollution control devices.
From the foregoing, it is to be appreciated that the novel
composition and methods are efficient (high mercury uptake
capacity) and economical (cost effective) at substantially removing
mercury from mercury-containing fluids, including mercury from a
wide range of flue gases. The estimated material costs of the
oxidative sorbent compositions made from silicates and a spent
waste etching solution containing CuCl.sub.2.2H.sub.2O from the
Printed Circuit Board industry is less than half of the cost of
activated carbon. The oxidative sorbents are expected to result in
a significant cost reduction in mercury control (>50%) while
having a negligible effect on fly ash byproduct sales and use.
Although a particular embodiment of the invention has been
described in detail for purposes of illustration, various
modifications may be made without departing from the spirit and
scope of the invention. Accordingly, the invention is not to be
limited, except as by the appended claims.
* * * * *